1.\" Copyright (c) 2001,2002 John H. Baldwin <jhb@FreeBSD.org> 2.\" All rights reserved. 3.\" 4.\" Redistribution and use in source and binary forms, with or without 5.\" modification, are permitted provided that the following conditions 6.\" are met: 7.\" 1. Redistributions of source code must retain the above copyright 8.\" notice, this list of conditions and the following disclaimer. 9.\" 2. Redistributions in binary form must reproduce the above copyright 10.\" notice, this list of conditions and the following disclaimer in the 11.\" documentation and/or other materials provided with the distribution. 12.\" 13.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND 14.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 15.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 16.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE 17.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 18.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 19.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 20.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 21.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 22.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 23.\" SUCH DAMAGE. 24.\" 25.\" $FreeBSD$ 26.\" 27.Dd March 22, 2001 28.Dt CRITICAL_ENTER 9 29.Os 30.Sh NAME 31.Nm cpu_critical_enter , 32.Nm cpu_critical_exit , 33.Nm critical_enter , 34.Nm critical_exit 35.Nd enter and exit a critical region 36.Sh SYNOPSIS 37.In sys/param.h 38.In sys/proc.h 39.In sys/systm.h 40.In machine/critical.h 41.Ft void 42.Fn cpu_critical_enter "void" 43.Ft void 44.Fn cpu_critical_exit "void" 45.Ft void 46.Fn critical_enter "void" 47.Ft void 48.Fn critical_exit "void" 49.Sh DESCRIPTION 50These functions are used to prevent preemption in a critical region of code. 51All that is guaranteed is that the thread currently executing on a CPU will 52not be preempted. 53Specifically, a thread in a critical region will not migrate to another 54CPU while it is in a critical region. 55The current CPU may still trigger faults and exceptions during a critical 56section; however, these faults are usually fatal. 57.Pp 58The 59.Fn cpu_critical_enter 60and 61.Fn cpu_critical_exit 62functions provide the machine dependent disabling of preemption, normally 63by disabling interrupts on the local CPU. 64.Pp 65The 66.Fn critical_enter 67and 68.Fn critical_exit 69functions provide a machine independent wrapper around the machine dependent 70API. 71This wrapper currently saves state regarding nested critical sections. 72Nearly all code should use these versions of the API. 73.Pp 74Note that these functions are not required to provide any inter-CPU 75synchronization, data protection, or memory ordering guarantees and thus 76should 77.Em not 78be used to protect shared data structures. 79.Pp 80These functions should be used with care as an infinite loop within a 81critical region will deadlock the CPU. 82Also, they should not be interlocked with operations on mutexes, sx locks, 83semaphores, or other synchronization primitives. 84.Sh EXAMPLES 85This example demonstrates the use of 86.Fn critical_enter 87and 88.Fn critical_exit 89to guarantee atomic access to the DMA controller. 90.Bd -literal -offset indent 91int 92isa_dmastatus(int chan) 93{ 94 u_long cnt = 0; 95 int ffport, waport; 96 u_long low1, high1, low2, high2; 97 98 ... 99 critical_enter(); 100 outb(ffport, 0); 101 low1 = inb(waport); 102 high1 = inb(waport); 103 outb(ffport, 0); 104 low2 = inb(waport); 105 high2 = inb(waport); 106 critical_exit(); 107 ... 108} 109.Ed 110.Sh HISTORY 111These functions were introduced in 112.Fx 5.0 . 113